This process can be applied to all recent GM cars except those with computerized gauges.
1) Locate the fuel sender feed wire near the
fuel tank. You're looking for a single tan colored wire. With
the tan wire disconnected at the fuel tank, the fuel gauge should
read past full with the ignition on. Wait a few moments as some
fuel gauges take a long time to respond. Touch the tan wire from
the body to any convenient ground and the gauge should read empty.
If not, you have a wiring problem or a bad gauge.
2) If the gauge responds correctly, the gauge
and wiring are OK. Next use a multi-meter to measure resistance
to ground of the sender wire connection on the top of the fuel
sender or the tan wire from the top of the fuel tank. Measurements
should track the fuel in tank.
Full - 84-88 ohms
Half - 40 ohms, give or take
Empty - 0-2 ohms
If this doesn't check, then sender or wiring on top of the tank
is bad or the sender not adequately grounded. Senders are typically
grounded by a black wire which is welded to the sender and attached
to the body with a sheet metal screw.
3) If the sender checks OK but gauge and wiring
don't, clean the connections, reconnect the sender wiring and
separate the Fisher connector (located just outboard of the fuse
block under the dash). The gauge should then read past full. Ground
the tan wire in the dash side of the Fisher connector and the
gauge should read empty. If not, you probably have a bad gauge
or possibly a dash wiring problem. Go to Step 5.
4) If the gauge checks OK, then make the same
resistance checks to the tan wire in the body side of the Fisher
connector. If the readings are different than those at the sender,
body wiring has a problem and requires detailed inspection. If
they look OK, then the Fisher connector is probably dirty.
5) Clean and reconnect Fisher connector, pull
the connector off the back of the gauge and make the same resistance
checks to the tan wire. If they don't check, you have a dash wiring
problem. If they check OK, your gauge is bad. Gauges can be bench-checked
but this is best left to a specialist.